FIG. 6 is a cross-sectional view showing a relief valve 1 according to a prior art. The relief valve 1 is equipped in a pressure vessel and used for releasing a gas from the pressure vessel in an emergency situation such as occurrence of a fire in which ambient temperature rises. The relief valve 1 is formed by providing a piston 6, a pressing spring 7 and a fuse metal 8 in the interior of a housing 5 provided with a valve passage 4 connecting a primary port 2 and a secondary port 3 to each other. The piston 6 is provided at a closed position for closing the valve passage 4 and prevented from being displaced to an open position for opening the valve passage 4 by the fuse metal 8. The piston 6 receives a pressing force in an opening direction, which is caused by the gas guided through the primary port 2 and the pressing spring 7 [FIG. 6(1)]. With such a configuration, under normal condition, the valve passage 4 is closed, while under emergency condition, the fuse metal 8 is melted and thus the piston 6 is displaced so as to open the valve passage 4, thereby outputting the gas therefrom [FIG. 6(2)].
FIG. 7 is a cross-sectional view showing a relief valve 10 according to another prior art. The relief valve 10 in FIG. 7 is similar to the relief valve 1 in FIG. 6 and therefore only differences will be described. The relief valve 10 in FIG. 7 is configured in such a manner that a pressure cancel chamber 11 is formed on the opposite side of the primary port 2 with respect to the piston 6 and a through hole 12 formed in the piston 6 connects the primary port 2 to the pressure cancel chamber 12 [FIG. 7(1)]. In this manner, the pressing force applied to the fuse metal 8 by a primary pressure p1 is cancelled. Output of the gas under emergency condition is performed in the same manner as in the relief valve 1 shown in FIG. 6 [FIG. 7(2)] (For example, see Patent document 1).
Patent document 1: Japanese Laid-Open Patent Publication No. 2001-317645